OK Tony, If the "bellmouth" on the hose is about 4-3/8 inches in diameter, the area inside would be about 15 sq inches. That means that a minimum of 1 psi would hold a 15 lb bowling ball, therefore, it would be about 28 inches of water. Are we going to start flowing bowling balls?

the floating pressure drop flow bench would work and be useful, providing the minimum depression at maximum valve lift/maximum flow for each test port, was still at least 28". so one still needs a bench with enough vacuum motors/power to pull 28" at the max cfm the test piece can flow. you can buy the software and make the orifice plates, to convert it to precise cfm readings at 28" if you so wish- and while using the floating pressure drop method, the manometer can fluctuate from 28" up to whatever the bench will pull at minimum valve lift opening of .050"- but it can't fall below 28" any time during the test- otherwise a critical correlation is lost.

the floating pressure drop idea, is not an excuse, for utilizing flow benches that are underpowered, and can't pull 28" at max flow/max valve lift on the test piece. it's also not a good reason to buy software to convert the low pressure drop figures, to the 28" standard. that's false economy, because there are older used flow benches for sale now that flow 400 cfm at 28", for only $2000. more important, it's inaccurate. why waste your time ?

if you build a flow bench, build one that can flow a minimum of 28" with any port you may have to flow on it, at max valve lift.or buy a lesser bench and add more vacuum motors to it, to get to that standard. if you want to save some time, or money, you can still use the floating pressure drop method, providing the bench can pull 28" when you get to the end of the test, and have the valve wide open at max lift.

I had some big heads flowed for a V8, that required the operator to switch over to the next higher scale on an SF600, because the heads pulled up over 300 cfm at 28" at max lift. but I got numbers that I know are bona fide and can be trusted, not bogus numbers. strive for accurate results.

the 28" standard is there for a very good reason- it was developed 65 years ago, before most of us were even born. it was found through a lot of elaborate work, and back to back testing using flow bench and dyno. what most people don't refer to, and don't read carefully, is all the reasons WHY it was chosen. anyone considering using this method, first read Smokey's Power Secrets, and re-read his reasoning for choosing 28" carefully. no one in these threads refers to the whole story. there were a number of very crucial reasons 28" was chosen:

1. at low flow rates such as 8", big changes were made to a port that definitely improved flow, but these changes did not show up as improvements or detriments on the flow bench at 8". a higher depression had to be used for the improvements to actually show up as higher flow bench readings. this means your bench isn't telling you much at low pressure drops.

2. Smokey was already flowing complete engines and pulling through the oil pan for intake, and exhaust header for exhaust, and his flow tests were quite elaborate. he was looking for a standard whereby he could just work with bare heads/ports, and save a lot of time

3. he wanted to see a direct relationship, where flow improvements found with bare heads on the bench, would have a direct correlation to increased horsepower on the dyno. he would port heads, flow them at ever increasing pressures, and when the flow improvement showed up at the higher flow rates on the bench, immediately assembled the engine, and dyno tested it, to see if there was a corresponding horsepower increase. that flow bench to dyno HP correlation, only occurred at the higher flow rates on the bench. every time flow was increased at 28" on the bench, the engine made more HP on the dyno. direct link.

so basically he wanted to make sure, if he trusted a flow bench to flow bare heads, that the increased flow he saw on the bench, actually made more HP on the dyno- otherwise why port heads, find an alleged flow improvement, and flow them on an expensive bench, if the engine didn't make more power as a result ?

28" was the best all around number that accomplished that. it's the "no bs" number. there was a ton of man hours, expense, and proof work put into that discovery.

one major reason cited for this "floating depression" is blowdown/overlap pressure in a running engine, is very high when both intake/exhaust are open. if you look at the pressure chart for a running engine, you'll see the spike in depression during overlap is only for a brief amount of time, compared to the rest of the intake cycle when the intake valve is open.

while it may ease your conscience to make a cheapie flow bench with vacuum cleaner motors, and use relatively cheap software to convert to the standard, the numbers are not going to be accurate, or as accurate, as the 28" standard on a bench that can actually pull 28" at max valve lift.

if you build a flowbench, build one with enough motors in it to pull 28" minimum. you can still use the floating pressure drop method, and if it shows a favorable depression change after you port the head (i.e. lower depression), you know it's improving, and will correlate to more HP on the dyno- whether you choose to convert the readings cfm at 28" or not.

for some reason the low dollar bench, and the relatively higher cost conversion software, became the focus of the floating pressure drop method- rather than accuracy and usefulness.

IMHO repeatability within 1% in cfm, is not of the utmost importance. If I take 5 heads and flow them all, I want the best one to show the highest readings, even if it's still in inches of H20 off the manometer, and not converted to cfm at 28" standard yet. to do that, you need at least 28" of pressure drop on the port at all times, higher is ok if using the floating pressure drop method. otherwise, if the pressure is allowed to drop below 28", or way down to 8", the better port may not show up as better, and the port that can make the most HP on a dyno, will not show up as the best either, while on the flow bench. it would make the entire test irrelevant.

the 28" standard, is what links the increased flowbench results, to actual increased dyno HP, and makes it believable, and valid.

I just read through most of the posts on here. I am building a PTS Flow bench because it makes good common sense and logic is on its side. To me Bruce has designed a no BS flow bench that is world class. End of discussion.

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